I have a question regarding installing some outdoor lights.
Since I already have an outdoor light connected to a switch & dedicated breaker is it possible to add another 10-15 lights to it?

The switch is 12A/240v & the lights are 40w each.

wires = 1.5mm

You don't mean you plan to connect patio lights to a 240 line do you? I guess I should ask what country are you?

This is what the elctrician suggested in order to avoid drilling through the house wall. I'm from Ireland. Are there any other alternatives?

I better be quiet on this since I know little about your power in Ireland.

John,

Amps = watts divide by volts, so one lamp is 40 watts / 240 volts =.167 amps

10 lamps will draw 1.67 amps,15 will draw 2.505 amps

What you don't tell is what is load existing load on the 12 amp switch. If it only one light fixture, I would say the circuit can handle the added 2.505 amps.

These calcs are easy for us to review, Ohm's Law works on both sides of the pond.

What we don't know is the Ireland's electric code, specifically derating of the ampacity of wire and devices. To explain, here a 20 amp circuit can only handle 80%, so wire or circuit can only handle 16 amps. We don't know what the max allowed amps is on a 12 amp circuit there.

With a bit of research I believe Ireland follows BS6500 Building Standards, and 1.5 mm wire is rated 16 Amps. What we don't know is type of insulation, as this can change the rating of wire.

I am a bit confused thou. Since you have an electrician, shouldn't he/she be doing these calcs, as they should be up to date and well versed with local codes, and of course , Ohm's Law.

While running the wire in a fashion that will save labor or materials, the circuit should be looked at and confirmed to handle the new load. No guessing allowed with electricity. We will help where we can, but need to refer you to your electrician, or one that knows local codes.

Get back with any other questions, and let u know how you make out.

All the best.